1. Field of the Invention
The present invention relates to a tone correction technique for an image.
2. Description of the Related Art
Conventionally, a method is known for executing tone correction by analyzing the luminance histogram and object information of an acquired image so as to obtain an image with a favorable brightness and contrast.
In particular, in a so-called backlight scene in which the brightness of a main object is considerably darker than that of a background, since a main object part of a sensed image becomes dark, it is effective to apply tone correction to such images. Also, as for a so-called “highlight-detail loss” in which the brightness of a background becomes considerably brighter, the following method is available. Initially, in a stage of exposing an image sensor, an image is sensed to have an exposure value relatively darker than a proper exposure value so as to suppress a highlight-detail loss. Then, using a correction table (tone curve) which converts the luminance values of obtained image data to predetermined output luminance values, tone correction is applied so that the levels of luminance values of a region which is sensed to be relatively darker are corrected to proper levels, and the levels of luminance values of a region which is sensed to be already bright remain unchanged. This method is known to be effective for digitally enhancing the dynamic range of the image sensor (for example, see Japanese Patent Laid-Open No. 2007-124604).
A method of obtaining a so-called shading-like effect by applying tone correction using low-frequency component signals of luminance components of an image has been proposed. With this method, tone correction that makes an object, which is sensed to be darker, be brighter while maintaining the contrast of an image can be executed (for example, see Japanese Patent Laid-Open No. 2008-072604).
On the other hand, since auto exposure control determines an exposure value to obtain a predetermined luminance level even when a frame includes a white part like a white wall or snow scene, an object which is originally white may often be sensed to have a color closer to gray. As is known, by applying tone correction to an image of such scene so that when there are a few or no high-luminance components, an output luminance region to be assigned to an input high-luminance region is narrowed down to skip tone assignment to an unnecessary luminance region, an effect of sensing a white object intact is obtained.
However, when tone correction is applied using the tone curve so that an image dark part is corrected to be brighter and correction for an originally bright part is suppressed, as described in Japanese Patent Laid-Open No. 2007-124604, the contrast in an intermediate luminance region between a main object and background is lost, resulting in a flat image.
On the other hand, when tone correction is applied using low-frequency component signals of luminance components of an image, as described in Japanese Patent Laid-Open No. 2008-072604, too much contrast is obtained depending on scenes, resulting in an unnatural image.